1. Field of the Invention
The invention relates to underwater sound technology and, in particular, concerns sonar systems with multiple acoustic beams being formed and steered by frequency division.
2. Description of the Related Art
Nearly all under-water vehicles, whether manned or unmanned, are equipped with an ahead-look sonar (ALS). Common applications include obstacle avoidance, mine detection, and rendezvous and docking capabilities. In general, these sonars use electronically beamformed arrays, although some use simpler systems in which multiple, separate directional hydrophones are used to provide multiple preformed beams. The electronically beamformed systems offer improved performance but are substantially more costly because of the complexities of the beamforming circuitry. A typical multi-beam sonar forms about 100 beams over an angular sector of about 150 degrees. The beamforming circuitry for this sonar requires about 100 receiver channel amplifiers to raise the received signal to a level sufficient for digital beamforming of the 100 beams. Because of this receiver amplifier complexity which is proportional to the beam resolution (beamwidth) and the number of beams formed, the multiple physical beam approach is typically limited to a relatively small number of low resolution beams and may be relatively heavy because of the excess of ceramic material in the multiple hydrophones.
Another method of forming multiple beams without an electronic beamformer is the use of an acoustic lens with a multi-element retina of small hydrophones. Although appealing in principle, lens arrays have proven difficult in practice due to issues such as temperature instability and toxicity of fluid materials and shear wave effects in solid lenses. Lens sonars also have problems in terms of the size and weight of the physical beamformer.
Thus, present beamforming techniques have practical cost, size and weight deficiencies. These are particularly important in applications such as small, low cost unmanned under-water vehicles (UUVs) where size and weight are at a premium and the cost of individual subsystems such as sonars preferably needs be kept low.
A frequency scanning technique has been used in radar for many years. Instead of fixed phase shifts between elements, however, the radar implementations use long delay lines between antenna elements or radiating slots in a dispersive delay line. The typical application is to provide vertical scanning of an array where azimuthal scanning is provided by either mechanical rotation or another electronic phase shifting technique.
Hence, there is a need for a sonar system that permits sequential scanning through multiple beams or forming multiple simultaneous acoustic beams. There is need for such sonar system to be implemented in a simple cost and size/weight effective manner.
In one aspect, the aforementioned needs are satisfied by a sonar system for forming a steerable underwater acoustic beams. The system comprises an array of acoustic transducers and a beamforming system that associates a signal to each of the transducers to form an acoustic beam with a direction. The signal is phase shifted by a selected fixed amount relative to a signal assigned to the adjacent transducer and the direction of the acoustic beam is determined by the frequency of the signals. The beamforming system is adapted to vary the frequency of the signals so as to permit steering of the acoustic beam.
In one embodiment, the beamforming system comprises a transmitter that supplies signals to the array so as to form a transmitted acoustic beam. In another embodiment, the beamforming system comprises a receiver that receives signals from the array that results from a received acoustic beam. In another embodiment, the beamforming system comprises a transmitter that supplies signals to the array so as to form a transmitted acoustic beam, and a receiver that receives signals from the array that results from a received acoustic beam.
In one embodiment, a formula cos xcex8=(xcex94xcfx86/2xcfx80)(c/fd) represents a relationship between the direction of the acoustic beam and the frequency, where xcex8 represents a direction angle relative to a plane defined by the transducers, xcex94xcfx86 represents a phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, f represents the frequency of the signals, and d represents spacing between the adjacent transducers, wherein the phase shift xcex94xcfx86 is selected to be a substantially constant value andy the direction angle xcex8 is varied by varying the frequency about a center frequency f0. The phase shift xcex94xcfx86 is selected such that a signal associated with a given acoustic transducer is a simple linear combination of signals proportional to cos xcfx89t and sin xcfx89t, where xcfx89=2xcfx80f and t represents time. In one implementation, the phase shift xcex94xcfx86 between the adjacent acoustic transducers is selected to be approximately xcfx80/2 radian such that repeating sets of four acoustic transducers can be associated by a sequence of signals proportional to cos xcfx89t, sin xcfx89t, xe2x88x92cos xcfx89t, and xe2x88x92sin xcfx89t. In another implementation, the phase shift xcex94xcfx86 between the adjacent acoustic transducers is selected to be approximately 3xcfx80/4 radian such that repeating sets of eight acoustic transducers can be associated by a sequence of signals proportional to cos xcfx89t, xe2x88x921/{square root over (2)} cos xcfx89t+1/{square root over (2)} sin xcfx89t, xe2x88x92sin xcfx89t, 1/{square root over (2)} cos xcfx89t +1/{square root over (2)} sin xcfx89t, xe2x88x92cos xcfx89t , 1/{square root over (2)} cos xcfx89txe2x88x921/{square root over (2)} sin xcfx89t, sin xcfx89t, and xe2x88x921/{square root over (2)} cos xcfx89txe2x88x921/{square root over (2)} sin xcfx89t. In one implementation, the frequency f of the signals is varied in a range of approximately 0.75f0 to approximately 1.25f0.
In another aspect, the aforementioned needs are satisfied by an underwater sonar system comprising an array of acoustic transducers and a beamforming system that simultaneously associates signals with a range of frequencies to the transducers. A signal to a given transducer is phase shifted by a selected fixed amount relative to a signal assigned to the adjacent transducer. The phase shifted signals with the range of frequencies form an acoustic signal with a range of directions. A given direction of propagation within the range of directions corresponds to a specific frequency of the signals within the range of frequencies.
In one embodiment, the beamforming system comprises a broadband transmitter that simultaneously supplies signals with a range of frequencies to the array so as to form transmitted acoustic signals with a range of directions. In another embodiment, the beamforming system comprises a receiver having a spectrum analyzer that simultaneously processes signals from the array that result from received acoustic signals from a range of directions. In another embodiment, the beamforming system comprises a broadband transmitter and a receiver having a spectrum analyzer. The broadband transmitter simultaneously supplies signals with a range of frequencies to the array so as to form transmitted acoustic signals with a range of directions, and the spectrum analyzer simultaneously processes signals from the array that result from received acoustic signals from a range of directions.
A formula cos xcex8=(xcex94xcfx86/2xcfx80)(c/fd) represents a relationship between the direction of the acoustic signal and the frequency, where xcex8 represents a direction angle relative to a plane defined by the transducers, xcex94xcfx86 represents a phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, f represents the frequency of the signals, and d represents spacing between the adjacent transducers. The phase shift xcex94xcfx86 is selected to be a substantially constant value and the direction angle xcex8 is varied by varying the frequency f. Preferably, the phase shift xcex94xcfx86 is selected such that a signal associated with a given acoustic transducer is a simple linear combination of signals proportional to cos xcfx89t and sin xcfx89t, where xcfx89=2xcfx80f and t represents time. In one implementation, the phase-shift xcex94xcfx86 between the adjacent acoustic transducers is selected to be approximately xcfx80/2 radian such that repeating sets of four acoustic transducers can be associated by a sequence of signals proportional to cos xcfx89t, sin xcfx89t, xe2x88x92cos xcfx89t, and xe2x88x92sin xcfx89t. In another implementation, the phase shift xcex94xcfx86 between the adjacent acoustic transducers is selected to be approximately 3xcfx80/4 radian such that repeating sets of eight acoustic transducers can be associated by a sequence of signals proportional to cos xcfx89t, xe2x88x921/{square root over (2)} cos xcfx89t+1/{square root over (2)} sin xcfx89t, xe2x88x92sin xcfx89t, 1/{square root over (2)} cos xcfx89t+1/{square root over (2)} sin xcfx89t, xe2x88x92cos xcfx89t, 1/{square root over (2)} cos xcfx89t xe2x88x921/{square root over (2)} sin xcfx89t, sin xcfx89t, and xe2x88x921/{square root over (2)} cos xcfx89txe2x88x921/{square root over (2)} sin xcfx89t.
In yet another aspect, the aforementioned needs are satisfied by a method of using an underwater sonar system having an array of acoustic transducers. The method comprises associating signals having a frequency component to the transducers. A signal associated with a given transducer is phase shifted by a selected fixed amount relative to a signal assigned to the adjacent transducer such that the phase shifted signals form an acoustic beam having a direction. The method further comprises controlling the directionality of the acoustic beam by manipulating the frequency component of the signals.
In one implementation, associating the signals to the transducers comprises associating the transducers with signals with a frequency f such that a formula cos xcex8=(xcex94xcfx86/2xcfx80)(c/fd) represents a relationship between the direction of the acoustic beam and the frequency, where xcex8 represents a direction angle relative to a plane defined by the transducers, xcex94xcfx86 represents the selected fixed phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, and d represents spacing between the adjacent transducers. Preferably, associating the signals to the transducers comprises selecting the phase shift xcex94xcfx86 such that a signal associated with a given transducer is a simple linear combination of signals proportional to cos xcfx89t and sin xcfx89t, where xcfx89=2xcfx80f and t represents time. The phase shift xcex94xcfx86 between the adjacent acoustic transducers may be selected to be approximately xcfx80/2 radian such that repeating sets of four acoustic transducers can be associated by a sequence of signals proportional to cos xcfx89t, sin xcfx89t, xe2x88x92cos xcfx89t, and xe2x88x92sin xcfx89t. Alternatively, the phase shift xcex94xcfx86 between the adjacent acoustic transducers may be selected to be approximately 3xcfx80/4 radian such that repeating sets of eight acoustic transducers can be associated by a sequence of signals proportional to cos xcfx89t, xe2x88x921/{square root over (2)} cos xcfx89t+1/{square root over (2)} sin xcfx89t, xe2x88x92sin xcfx89t, 1/{square root over (2)} cos xcfx89t+1/{square root over (2)} sin xcfx89t, xe2x88x92cos xcfx89t, 1/{square root over (2)} cos xcfx89txe2x88x921/{square root over (2)} sin xcfx89t, sin xcfx89t, and xe2x88x921/{square root over (2)} cos xcfx89txe2x88x921/{square root over (2)} sin xcfx89t.
In one implementation, associating the signals with the transducers comprises associating a narrowband signal with the transducers and varying the frequency of the narrowband signal to change the direction of the acoustic beam. Associating the narrowband signal with the transducers may comprise supplying the narrowband signal to the transducers wherein the signal applied to the transducers results in an outgoing acoustic beam. Alternatively, associating the narrowband signal with the transducers may comprise receiving an echo signal from the transducers wherein the echo signal result from an echo that impinges on the transducers. Alternatively, associating the narrowband signal with the transducers may comprise supplying the narrowband signal to the transducers to yield an outgoing acoustic beam, and receiving an echo signal from the transducers that result from an incoming echo.
In another implementation, associating the signals with the transducers comprises associating a broadband signal having a range of frequencies with the transducers such that corresponding acoustic beams have a range of directions. Associating the broadband signal with the transducers may comprise simultaneously providing a broadband signal to the transducers so as to yield a plurality of outgoing acoustic beams having a range of directions. Alternatively, associating the broadband signal with the transducers may comprise simultaneously receiving a broadband echo signal from the transducers that result from a plurality of incoming echoes. Alternatively, associating the broadband signal with the transducers may comprise simultaneously proving a broadband signal to the transducers to yield a plurality of outgoing acoustic beams having a range of directions, and simultaneously receiving a broadband echo signal from the transducers that result from a plurality of incoming echoes.
In yet another aspect, the aforementioned needs are satisfied by a method of scanning an angular sector underwater using an array of acoustic transducers. The method comprises forming a plurality of acoustic beams wherein each acoustic beam is formed by associating signals to the array of acoustic transducers such that a signal associated a given transducer is phase shifted by a selected fixed amount relative to a signal assigned to the adjacent transducer. The direction of each acoustic beam depends on the frequency of the signals. The method further comprises varying the frequency of signals corresponding to each acoustic beam so as to vary the direction of the acoustic beam, thereby allowing the acoustic beam to sweep a range of direction angles. The frequency is selected for each acoustic beam such that resulting ranges of direction angles cover the angular sector.
In one implementation, a formula cos xcex8=(xcex94xcfx86/2xcfx80)(c/fd) represents a relationship between the direction of the acoustic beam and the frequency f, where xcex8 represents a direction angle relative to a plane defined by the transducers, xcex94xcfx86 represents the selected fixed phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, and d represents spacing between the adjacent transducers. Preferably, the phase shift xcex94xcfx86 is selected such that a signal associated with a given transducer is a simple linear combination of signals proportional to cos xcfx89t and sin xcfx89t, where xcfx89=2xcfx80f and t represents time. The phase shift xcex94xcfx86 between the adjacent acoustic transducers may be selected to be approximately xcfx80/2 radian such that repeating sets of four acoustic transducers can be associated by a sequence of signals proportional to cos xcfx89t, sin xcfx89t, xe2x88x92cos xcfx89t, and xe2x88x92sin xcfx89t. Alternatively, the phase shift xcex94xcfx86 between the adjacent acoustic transducers may be selected to be approximately 3xcfx80/4 radian such that repeating sets of eight acoustic transducers can be associated by a sequence of signals proportional to cos xcfx89t, xe2x88x921/{square root over (2)} cos xcfx89t+1/{square root over (2)} sin xcfx89t, xe2x88x92sin xcfx89t, 1/{square root over (2)} cos xcfx89t+1/{square root over (2)} sin xcfx89t, xe2x88x92cos xcfx89t, 1/{square root over (2)} cos xcfx89txe2x88x921/{square root over (2)} sin xcfx89t, sin xcfx89t, and xe2x88x921/{square root over (2)} cos xcfx89txe2x88x921/{square root over (2)} sin xcfx89t.
In one implementation, associating the signals with the transducers comprises associating a narrowband signal with the transducers and varying the frequency of the narrowband signal to sweep the acoustic beam within the range of direction angles. Associating the narrowband signal with the transducers may comprise supplying the narrowband signal to the transducers wherein the signal applied to the transducers results in an outgoing acoustic beam. Alternatively, associating the narrowband signal with the transducers may comprise receiving an echo signal from the transducers wherein the echo signal result from an echo that impinges on the transducers. Alternatively, associating the narrowband signal with the transducers may comprise supplying the narrowband signal to the transducers to yield an outgoing acoustic beam, and receiving an echo signal from the transducers that result from an incoming echo.
In yet another aspect, the aforementioned needs are satisfied by a sonar system for forming a steerable underwater acoustic beams. The system comprises an array of acoustic transducers and a beamforming system that associates a signal to each of the transducers to form an acoustic beam with a direction. The signal is phase-shifted by a selected phase relative to a signal assigned to the adjacent transducer and the direction of the acoustic beam is determined by a combination of the phase and the frequency of the signals. The beamforming system is adapted to vary the frequency of the signals for a given phase so as to permit steering of the acoustic beam.
In one embodiment, a formula cos xcex8=(xcex94xcfx86/2xcfx80)(c/fd) represents a relationship of the direction of the acoustic beam to phase and frequency, where xcex8 represents a direction angle relative to a plane defined by the transducers, xcex94xcfx86 represents a phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, f represents the frequency of the signals, and d represents spacing between the adjacent transducers, wherein the phase xcex94xcfx86 is selected to direct the beam in a general desired first direction, and the frequency f is varied to vary the direction of the beam about the first direction.
In one embodiment, the beamforming system comprises a transmitter that supplies signals to the array so as to form a transmitted acoustic beam. In another embodiment, the beamforming system comprises a receiver that receives signals from the array that results from a received acoustic beam. In another embodiment, the beamforming system comprises a transmitter that supplies signals to the array so as to form a transmitted acoustic beam, and a receiver that receives signals from the array that results from a received acoustic beam.
In yet another aspect, the aforementioned needs are satisfied by a method of using an underwater sonar system having an array of acoustic transducers. The method comprises associating signals having a frequency component and a phase component to the transducers. A signal associated with a given transducer is phase-shifted by a selected phase relative to a signal assigned to the adjacent transducer. The method further comprises controlling the directionality of the acoustic signal by selecting a first direction of the acoustic signal as determined by the selected phase and varying the direction of the acoustic beam about the first direction by manipulating the frequency component of the signals.
In one implementation, associating the signals to the transducers comprises associating the transducers with signals with a frequency f and a the phase xcex94xcfx86 such that a formula cos xcex8=(xcex94xcfx86/2xcfx80)(c/fd) represents a relationship of the direction of the acoustic signal to the phase and frequency, where xcex8 represents a direction angle relative to a plane defined by the transducers, xcex94xcfx86 represents the selected phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, and d represents spacing between the adjacent transducers.
In yet another aspect, the aforementioned needs are satisfied by a sonar system for forming a steerable underwater acoustic beams. The system comprises an array of acoustic transducers and a beamforming system having a set of beamformers. The beamformers associate a plurality of signals to the transducers to form an acoustic beam with a direction. Each of the signals is phase-shifted by a selected phase relative to a signal assigned to the adjacent transducer. The direction of the acoustic beam is determined by a combination of the phase and the frequency of the signals. The beamforming system is adapted to vary the frequency of the signals for a given phase so as to permit steering of the acoustic beam. A subset of the beamformers is connected to more than one repeating subsets of the transducers such that each beamformer associates a signal having an assigned phase and frequency to more than one transducer. This allows the total number of beamformers to be less than the number of transducers in the array.
In one embodiment, a formula cos xcex1=(xcex94xcfx86/2xcfx80)(c/fd) represents a relationship of the direction of the acoustic beam to phase and frequency, where xcex1 represents a direction angle relative to a normal to a plane defined by the transducers, xcex94xcfx86 represents a phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, f represents the frequency of the signals, and d represents spacing between the adjacent transducers. The phase xcex94xcfx86 is selected to direct the beam in a general desired first direction, and the frequency f is varied to vary the direction of the beam about the first direction. The phase xcex94xcfx86 is selected to be an integral fraction of 2xcfx80 radians to allow repeated duplication of signal assignments of the subset of the beamformers to the more than one subsets of the transducers.
In one embodiment, the array of transducers comprises a first line array. The spacing d is selected to be approximately half of the wavelength, and the phase xcex94xcfx86 is selected as 0, xcfx80/8, xcfx80/4, and xcfx80/8 radians progressively so as to allow progressive scanning about the different first directions as determined by the selected phases. The frequency is varied at each of the selected phases by approximately 67% of bandwidth about a center frequency such that the resulting sweepings of the beam about the first directions yield a generally seamless coverage of scanning that has a range of approximately 0 to 41.8 degrees with respect to the normal.
In one embodiment, the sonar system further comprises a second line array oriented perpendicularly to the first line array so as to form a cross shape to allow scanning in two dimensions.
In one embodiment, the beamforming system comprises a transmitter that supplies signals to the array so as to form a transmitted acoustic beam. In another embodiment, the beamforming system comprises a receiver that receives signals from the array that results from a received acoustic beam. In yet another embodiment, the beamforming system comprises a transmitter that supplies signals to the array so as to form a transmitted acoustic beam, and a receiver that receives signals from the array that results from a received acoustic beam.